Those enrolled in the study will be admitted to the NIH Clinical Center for a single dose of SCH 55700, injected into an arm vein. They will be monitored in the hospital for at least 72 hours for changes in eosinophil count and side effects of the injection. After discharge, laboratory tests will be done weekly for the first month, either at the Clinical Center or by the patient's local physician. Follow-up visits will then be scheduled monthly for 1 year or until the patient's eosinophil count returns to pre-treatment levels for 2 consecutive months. Follow-up visits will include a history, physical examination and blood tests, including studies on how immune cells and other substances in the blood activate or stimulate eosinophilia. A chest X-ray, electrocardiogram and pulmonary function tests will be done at 1, 3, 6 and 12 months to evaluate organ damage. Other tests may be done if medically indicated.